The present invention relates to an injection molding method and injection mold and especially to an injection mold with gas openings on the stationary or movable halves of the mold or from both sides for applying gas under pressure to the plastic mold.
Injection molding machines are used in shaping thermoplastic compositions. The preheating, plasticizing and shaping are all done in the injection molding machine where the thermoplastic is heated and plasticized and injected under pressure into the mold by means of a screwram which forces the material through a nozzle in to the mold. The molds are formed in two or more components which fit together to form a cavity therebetween. The mold is mounted to the injection molding machine and has a fixed portion and a moving portion. The mold has a movable portion which is connected to a moving platen of the molding machine for opening and closing the mold. When the mold is closed, the heated (plasticized) plastic is injected into the cavity, where it is allowed to cool to sufficiently harden the polymer before opening the mold by the moving of the movable portion of the injection mold. The polymer part or parts are then ejected from the mold which has formed an article of manufacture or a portion of an article of manufacture.
The present invention deals with a molding process in which the movable, stationary or both portions of the mold has gas injection ports fitted to each part of the mold to apply a nitrogen or other gas pressure to the one side or both sides of the part being molded during the molding process. The gas entry ports are covered either with a porous metal which is porous to the flow of the nitrogen gas but will block the entry of the more viscous plasticized polymer into the porous metal holes. The ports may also be sealed with poppet valves which are opened under the gas pressure which is applied sometime after the injection of the molten plastic into the mold cavity.
Gas and especially nitrogen has been commonly used in injection molding processes to apply a gas pressure through the injection nozzle for increasing the pressure behind the molten plastic being injected into the mold cavity. Two prior U.S. patents to Sauer, U.S. Pat. No. 4,295,811 and 4,309,380, show a blow molding process applying the gas into the thermoplastic articles through a porous metal section equipped to the core rod which is inserted into the polymer for expanding the polymer into a mold. These patents teach a different type of molding process from the present invention by using a porous metal for injecting the required air to form the blow molded part.
The aim of the present invention is to provide an injection mold and method which can apply a gas pressure to the outer surfaces of the molding and can apply the gas to a plurality of locations, at least one or more in each isolated mold portion, which gas pressure can be the same or can be varied within each section of the mold.